Filter for separating high frequencies from low frequencies



Jan. 24-, 1939. A. WIESSNER 2,144,950 FILTER FOR SEPARATINFI HIGH FREQUENCIES FROM LOW FREQUENCIES Filed July 10, 1957 Patented Jan. 24, 1939 UNITED STATES PATENT OFFICE FILTER FOR SEPARATING HIGH FREQUEN- CIES FROM LOW FREQUENCIES Application July 10, 1937, Serial No. 153,072 In Germany September 4, 1936 communications over a network which at the same time serves for the transmission of low frequency communications. For instance, high frequency carrier waves are impressed upon a telephone network in order to serve for the transmission of amusement or alarm communications or signals. In arrangements of this kind low frequency and high frequency must be separated from each other or brought together, as the case may be, by so-called separating filters. One such filter is disposed in the subscribers station and may therefore be designated as subscriber's filter and another is disposed in the exchange station or office and hence may be designated as ofiice filter.

It is also known to compose such an office filter of a condenser arranged to short-circuit the outgoing line wires in high frequency fashion, and 20 of two inductances included in the line wires.

Similarly, the subscriber's filter is composed inductances and short-circuiting condensers, both disposed in the low frequency wires, and of a filter to take the high frequency energy from the subscriber line.

In filters of this kind difiiculties arise through the fact that on the low frequency side also high frequency oscillations are produced which in each of the two line wires are in phase coincidence with 30 those occurring in the other wire and therefore require special separatory means to be provided.

Such disturbing oscillations arise for instance by the sparking that occurs on opening and closing contacts, as is the case for example with the 35 dials of talking stations. The necessity for such separatory means is undesirable because it involves that in the exchanges, for instance, many separatory filters have to be arranged side by side and because further the apparent resistance 40 of the lowpass filter is varied by the addition of means adapted to obviate disturbances.

In accordance with the invention the low-pass filter, included in the low frequency wires, is arranged also to suppress any disturbing oscillations which seek to arise in the two wires in both phase coincidence and phase opposition. This will be understood from the following description, reference being had to the drawing which is a. wiring diagram illustrating one embodiment of the invention. The arrangement here shown is applicable on both the subscriber side and exchange side. In the case represented a subscriber's arrangement is fitted therewith.

To this arrangement high frequency and at the same time low frequency is supplied over the wires a, b. The high frequency is conveyed over the high-pass filter l to a receiving arrangement such as a broadcast receiver 2, for instance. The low frequency is conducted to the low frequency subscribers station, for instance a telephone station, over inductances 3, 4 each included in one of the two wires 0., b and each of which has a core of high frequency iron, and over inductances 'l, 8 which have one core common to them. Between the wires a, b condensers 5, 6 are located 10 in order to short-circuit the high frequency. The low-pass filter thus provided prevents the high frequency, which is arriving on the wires a, b, from passing to the low-frequency subscribers station 9.

The coils 3, 4, I, 8 and condenser 5, 6 cause the high frequency to be effectually attenuated. As the coils 1, 8 have a common core, their leakage thus being very low, disturbing voltages in phase coincidence in the two wires do not meet with any inductive resistance. They flow over the condensers 5, 6 to earth. To eliminate disturbances in this way is favoured by the coils 3, 4 having separate cores and therefore offering a considerable resistance to such disturbing voltages. This arrangement thus acts in a very simple manner to combine the function of a low-pass filter with that of a filter circuit arranged to eliminate disturbances.

If filters of this construction. are employed on the exchange side, the high frequency is conveyed over the high-pass filter I to the line a, b which then is the out-going line, whilst the low frequency voltages are conducted to line a, b over the low-pass filter constituted by the parts 3 to 8. 36

What is claimed is:

1. In combination, means for generating high frequency and low frequency currents, a single pair of wires over which both said currents are transmitted, two line wires for the low frequency 40 current connected with said pair of wires, and a low-pass filter included in said line wires, said filter being adapted to suppress cophasal as well as oppositely phased high frequency disturbing oscillations in said line wires.

2. An arrangement according to claim 1, wherein the low-pass filter comprises inductances included in each of the said line wires and each having a core of its own, and further comprises two condensers for short-circuiting said line wires 50 in high frequency fashion, and also comprises inductances included in each of said line wires and having one core common to them.

ALFRED WIESSNER. 56 

